[Debug] Rename DebugDescription fallback property to lldbDescription (#75289)

  - **Explanation**:
From feedback, replace the name `_debugDescription`, which was confusing because of the 
underscore, with `lldbDescription`.
  - **Scope**:
This will not break existing code. Any existing uses of `_debugDescription` would see 
that the debugger does not produce a summary string, but would still display all 
fields/children of a value.
  - **Original PRs**:
#75305
  - **Risk**:
Early adopters of `_debugDescription` will not see their summary string shown.
  - **Testing**:
Swift tests, CI.
  - **Reviewers**:
@hborla, @stephentyrone, @DougGregor

Author: kastiglione